In large scale SMRs, approximately 50% of the thermal energy input into the firebox of a reformer is transferred to SMR reaction tubes and used to provide energy to drive the endothermic steam methane reforming reaction (CH4+H2O+206 kJ/molCO+3H2). Since the reforming reaction is generally carried out at a high temperature, e.g., 750° C. to 950° C., the temperature of a flue gas from the burner is generally at this temperature or above. The main usage of the high temperature flue gas is to generate steam through waste heat boilers. In typical SMRs, the flue gas can also be used to preheat combustion air; however, in prior art methods, the temperature of the flue gas must be maintained above the dew point of sulfuric acid to prevent formation of sulfuric acid. The temperature of the sulfuric acid dew point is normally 140° C. or above. The higher the sulfur content, the higher the sulfuric acid dew point. If the temperature of the flue gas is below the dew point of sulfuric acid, the equipment used in the low temperature range of the flue gas channel will encounter sulfuric acid corrosion. In order to eliminate the corrosion, stainless steel is used.
FIG. 1 is a layout of a conventional SMR system for producing hydrogen, carbon monoxide, or other useful products from hydrocarbon fuels such as natural gas (NG). As illustrated, fuel NG is directly fed to the burners of reformer 106 for use as fuel. Reformer 106 includes a combustion zone having a plurality of burners and a reaction zone containing a plurality of reforming tubes. A combustion air is heated through a pair of pre-heaters (cold air pre-heater (CAP) 116 and hot air pre-heater (HAP) 112) before being sent to the burners of reformer 106 for combustion of the fuel NG producing a flue gas stream.
A process natural gas (NG) is heated (not shown) and sent to hydrodesulfurization (HDS) unit 102 to remove sulfur from the natural gas. After that, the process gas is forwarded to pre-reformer 104 for breaking down long chain hydrocarbons in the natural gas into light hydrocarbons (e.g., methane), thereby forming a pre-reformed process NG. The pre-reformed process NG is fed to the reforming tubes in the reaction zone of reformer 106 under reforming conditions effective for converting methane within the process gas stream into carbon monoxide and hydrogen through the endothermic reaction (CH4+H2O+206 kJ/mol CO+3H2), thereby producing a syngas stream (H2+CO). The synthesis gas is converted to carbon dioxide (CO2) and hydrogen (H2) through shift reactor 108 forming a shifted gas.
The shifted gas is cooled further to ambient temperature before entering PSA unit 110. A product hydrogen stream and a PSA off-gas stream are then produced from PSA unit 110. The PSA off-gas, which includes methane (CH4), carbon dioxide (CO2), hydrogen (H2), and CO, is sent back to the burners of reformer 106 for use as fuel.
The flue gas from reformer 106, which typically has a temperature of about 1000° C., is delivered to different stages of heat exchangers, (i.e., hot air pre-heater (HAP) 112, flue gas boiler (FGB) 114 and cold air pre-heater (CAP) 116) to recapture heat from the flue gas at various temperatures. However, because the fuel NG includes sulfur, the flue gas must be maintained above the sulfuric acid dew point in order to avoid sulfuric acid condensation on the skin of the CAP and other low temperature apparatus in the system. This means that the energy of the flue gas below the sulfuric acid dew point is unused.
U.S. Pat. No. 8,187,363 issued to Grover, et al. discloses a method of preheating a PSA tail gas using low level waste heat in the flue gas or syngas prior to introduction into the SMR furnace combustion system. However, there is no mention of any problems associated with sulfuric acid formation. While Grover teaches recovering low level waste at temperatures between about 250° F. (˜120° C.) and about 300° F. (˜150° C.), these temperatures are only given as examples when using the syngas as the low level heat source. As such, Grover does not disclose cooling the flue gas to a temperature below the dew point of sulfuric acid, nor is there any discussion of potential problems associated with sulfur formation. Furthermore, no detailed implementation is disclosed.